Gas Phase Solvation of the Ammonium Ion by NH₃ and H₂O and Stabilities of Mixed Clusters NH₄⁺ (NH₃)n(H₂O)w

The gas phase equilibria[Formula: see text]and those leading to mixed clusters like[Formula: see text]were measured with a pulsed electron beam high pressure ion source mass spectrometer. The ion source contained pure ammonia or mixtures of ammonia and water vapor at pressures in the Torr range. Determination of the temperature dependence of the equilibrium constants led to the evaluation of ΔG 0 , ΔH 0 , and ΔS 0 values for the equilibria from n = 1 to 4 and w = 1 to 5. The ΔG 0 values for the NH 4 + (NH 3 ) n equilibria were in good agreement with previous determinations from this laboratory. Fair agreement was observed for the ΔH 0 and ΔS 0 values. Comparison with the corresponding results for NH 4 + (H 2 O) w showed that the ΔH n ,n−1 and ΔG 0 n,n−1 were larger than ΔH w ,w−1 and ΔG 0 w,w−1 . The difference was largest for the first step (1,0) and decreased progressively until a reversal with water values becoming larger occurred at the (5,4) step. The stronger hydrogen bonds of NH 3 to NH 4 + for low ligand numbers is explained by the greater basicity of NH 3 . As the ionic charge becomes dispersed and more distant stronger interactions are obtained with water which gives stronger H bonds in the absence of positive ionic charge. Breaks in the ΔH n ,n−1 values indicate existence of a relatively stable NH 4 + (NH 3 ) 4 symmetric ion. A much smaller and less distinct break is observed with pure water ligands.The mixed ammonia–water clusters show similar effects. The addition of a NH 3 molecule to a pure water ligand complex gives the strongest interaction. The addition of H 2 O to a pure ammonia cluster gives the weakest interaction. The above effect is strongest at lowest ligand numbers. The difference decreases gradually and becomes reversed for more than four ligands.